1. Field of the Invention
This invention relates to a method of correcting the gradation of a radiation image. This invention particularly relates to a method of adjusting the gradation of a reproduced image according to the recording material, development processing conditions, or the like, for the reproduced image in a radiation image recording and reproducing system using a stimulable phosphor.
2. Description of the Prior Art
A new system for recording and reproducing a radiation image by use of a stimulable phosphor is disclosed, for example, in U.S. Pat. Nos. 4,258,264; 4,315,318; 4,276,473; and 4,346,295. In this system, the stimulable phosphor is caused to absorb a radiation passing through an object and then stimulated by light energy to emit the radiation energy stored therein as light. The emitted light is detected and converted to an electric signal used for reproducing a visible image.
This radiation image recording and reproducing system using the stimulable phosphor is advantageous over the conventional radiography using a silver halide photographic material in that the image can be recorded over a very wide range of radiation exposure and further in that the electric signal used for reproducing the visible image can be processed as desired to improve the image quality for viewing, particularly for diagnostic purposes. In more detail, since the amount of light emitted upon stimulation after the radiation energy is stored in the phosphor varies over a very wide range in proportion to the amount of energy stored therein, it is possible to obtain an image having desirable density regardless of the amount of exposure of the phosphor to the radiation by converting the emitted light to an electric signal and changing the level of the electric signal to a desirable level corresponding to the desirable optical density of the image reproduced on a photographic film or the like.
In practical use, the range of the radiation exposure is limited by the dynamic range of the electric signal system used therefor. According to the experiments, it was possible to obtain images of desirable optical density even when the exposure value was changed over 3 orders, that is even when the dose of radiation was changed over the range of 1 to 1000.
This is very advantageous in practical use. For instance, when there are differences in exposure among a number of radiation images or there are over- or under-exposure images in a number of radiation images, these images can be processed to have the same level of optical density finally. Accordingly, mistakes made in the exposure step can be easily corrected. Further, the optical density of the finally obtained image can be selected simply by changing the level of the electric signal used for reproducing the image and, accordingly, it is possible to easily obtain an image having a desirable density for any kind of image. In other words, in the radiation image of the human body, the desirable density is different depending upon the kind of the image. In one kind or part of the human body, a high density is desirable for obtaining high diagnostic efficiency and accuracy and, in another kind or part, a low density is desirable. In the conventional radiography, the exposure is controlled to obtain the desirable density for the various kinds of images. In this sense, the above-mentioned system utilizing the stimulable phosphor and a gradation processing means is very advantageous. Furthermore, in the conventional radiography, a number of films of different sensitivity are prepared to be accommodated to a number of intensifying screens of different sensitivity. In the above-mentioned system, however, it is unnecessary to prepare a number of films of different sensitivity since one kind of film can be accommodated to various conditions of exposure and various sensitivities of the intensifying screens by later changing the level of the electric signal as desired.
As mentioned above, in the radiation image recording and reproducing system using a stimulable phosphor, the image information having a very wide range of level corresponding to the very wide range of exposure is once stored in the stimulable phosphor and is then read out and converted to an electric signal and finally converted to a visible image after processing the electric signal as desired. Therefore, the optical density of the finally obtained visible image or reproduced image can be controlled to the level desirable for viewing, particularly for diagnostic purposes. Thus, a radiation image having an improved image quality, particularly a high diagnostic efficiency and accuracy can be obtained.
However, when an image is actually reproduced on a recording material in the aforesaid radiation image recording and reproducing system, there occur changes in the conditions of the reproducing terminal sections, for example, changes in the kind of the recording material (light-sensitive material such as a photographic film), i.e. changes in the gradation (hard or soft), sensitivity or the like, changes in the development conditions, or changes in the power of the light source in the reproducing apparatus (due to deterioration with time). Therefore, it is not necessarily possible to obtain a reproduced image having a desirable density.
When the radiation image is used for medical diagnosis, the aforesaid problem interferes with the diagnosis, and it becomes impossible to correctly and accurately make a diagnosis. Particularly, diagnosis is difficult when a change in the symptoms of the patient with time has to be examined, since it is necessary to make the diagnosis by comparing many visible images reproduced under different conditions. In this case, if the sensitivity or the gradation changes among the images according to the reproducing conditions, it is very inconvenient from the viewpoint of diagnosis.